The American National Standard Institute has a standard for defining an input/output bus for interconnecting small computers and peripheral devices. The standard is referred to as the Small Computer System Interface (SCSI) Standard. It provides standardization of the defined command sets.
SCSI is a local I/O bus that can be operated over a wide range of data rates. The primary objective of the SCSI interface is to provide host computers with device independence within a class of devices. Thus, different disk drives, tape drives, printers, optical media drives, and other devices can be added to the host computers without requiring modifications to generic system hardware or software. Provision is made for the addition of special features and functions through the use of vendor unique fields and codes. Reserved fields and codes are provided for future standardization.
The SCSI interface uses logical rather than physical addressing for all data blocks. For direct-access devices, each logical unit may be interrogated to determine how many blocks it contains. A logical unit may coincide with all or part of a peripheral device.
The interface protocol includes provision for the connection of multiple initiators (SCSI devices capable of initiating an operation) and multiple targets (SCSI devices capable of responding to a request to perform an operation). Distributed arbitration (i.e., bus-contention logic) is built into the architecture of SCSI. A priority system awards interface control to the highest priority SCSI device that is contending for use of the bus. The time to complete arbitration is independent of the number of devices that are contending and can be completed in less than ten microseconds.
Regarding the logical characteristics of the interface, arbitration is defined to permit multiple initiators and to permit concurrent I/O operations. All SCSI devices are required to be capable of operating with the defined asynchronous transfer protocol.
SCSI commands are classified as mandatory, optional, or vendor unique. SCSI devices are required to implement all mandatory commands defined for the appropriate device type and may implement other commands as well. SCSI devices support commands that facilitate the writing of self-configuring software drivers that can "discover" all necessary attributes without prior knowledge of specific peripheral characteristics (such as storage capacity). Many commands also implement a very large logical block address space (2.sup.32 blocks), although some commands implement a somewhat smaller logical block address space (2.sup.21 blocks). Some commands have a consistent meaning for all device types.
Commands for direct-access devices (e.g. magnetic disk), sequential-access devices (e.g., magnetic tape), printer devices, processor devices, write-once devices (e.g., optical WORM disk), CD-ROM devices, scanner devices, optical memory devices, medium changer devices, and communications devices are included. The commands are unique to the device type, or they have interpretations, fields, or features that are specific for the device type. Thus, for example, although the WRITE command is used for several device types, it has a somewhat different form for each type, with different parameters and meanings. Therefore, it is set forth separately for each device type.
Communication on the SCSI bus per the SCSI Standard is allowed between only two SCSI devices at any given time. There is a maximum of eight SCSI devices allowed per SCSI bus. Each SCSI device has a unique ID number assigned from 0 to 7. When two SCSI devices communicate on the SCSI bus, one acts as an initiator and the other acts as a target. The initiator originates an operation and the target performs the operation. A SCSI device usually has a fixed role as an initiator or target, but some devices may be able to assume either role.
An initiator may address up to eight peripheral devices (i.e. Logical Units, LUNs) that are connected to a target. The target may be physically housed within a peripheral device. Up to eight SCSI devices can be supported on the SCSI bus. They can be any combination of initiators and targets provided there is at least one of each.
Certain SCSI bus functions are assigned to the initiator and certain SCSI bus functions are assigned to the target. The initiator may arbitrate for the SCSI bus and select a particular target. The target may request the transfer of COMMAND, DATA, STATUS, or other information on the DATA BUS, and in some cases it may arbitrate for the SCSI bus and reselect an initiator for the purpose of continuing an operation.
The SCSI architecture includes eight distinct phases: BUS FREE phase, ARBITRATION phase, SELECTION phase, RESELECTION phase, COMMAND phase, DATA phase, STATUS phase and MESSAGE phase. The Command, Data, Status and Message phases are collectively termed the information transfer phases. The SCSI bus can never be in more that one phase at any given time.
Although the SCSI Standard provides for communication between many types of devices within a given local bus of eight devices, it does not provide for communication between non-local SCSI buses or to non-SCSI devices. A SCSI device identifies itself to a computer by responding to an inquiry command with the device type field set to indicate what kind of device is attached (e.g., a printer, a magnetic disk, etc.) and with other fields set to indicate the appropriate standards it supports. Each device type has a set of SCSI commands which it supports (e.g., for a SCSI magnetic disk device there is supported a Read command, Write command, Format command and so forth).
There are many device types which can be connected to a SCSI bus such as printers, scanners, optical devices and processor devices. In the future, there likely will be many more. As previously stated, because of the SCSI architecture, there can be only eight typical devices connected to a SCSI bus. These devices or initiators separately send commands to the other devices, targets. As an example an initiator, which might be a SCSI adapter card in a computer, sends a Read command to a SCSI disk, i.e., the target, or sends a Print command to a printer, i.e., yet another target. The commands the initiator uses are usually the SCSI Standard commands for the given device. A computer can have several separate SCSI buses with eight local devices on each. A computer can also have devices which are not based on the SCSI Standard such as an Enhanced Small Device Interface (ESDI) disk or A printer connected to a parallel port.
One alternative method of accessing a device in another computer is to use a Local Area Network (LAN) system. This solution has a number of difficulties for the user. A LAN approach requires significant hardware and software investment and necessitates extensive system overhead. A network requires one network adapter per computer. An abundance of software is needed to implement the communication protocol and handle device sharing. Local Area Networks require the user to learn a new menu of commands. This is extra work for the end user and often adds significant delay in accessing the peripherals.
Accordingly a method is needed to conveniently expand beyond the SCSI standard imposed limit of communication with eight target devices and to provide access to SCSI devices on non-local buses and to non-SCSI devices attached to computers which share a common SCSI bus.